In VLSI fabrication, an ion implantation is primarily used to add dopant ions into the surface of silicon wafer. Ion implantation is a process in which energetic, charged atoms or molecules are directly introduced into a substrate. In order to benefit from the ability to control the number of impurities implanted into a substrate, it is necessary to know where the implanted atoms are located after implantation.
Ion implanters are one of the most complex systems used in the formation of VLSI. They contain many subsystem, including the following: (1) A feed source of material containing the species to be implanted; (2) An ion source, with its own power supply and vaccum pump, to ionize the feed gas and to produce a plasma (the ions are typically formed through collision with electrons from an arc discharge); (3) An ion extraction and analyzing device that selects only the ion species of interest according to their mass and rejects all others; (4) An acceleration tube which creates an acceleration field to increase the ion energy to a desired level; (5) A scanning system to distribute the ions uniformly over the target; (6) A high vacuum system for evacuating the source, acceleration column, and beam chamber; (7) A computer and a control system; (8) A system end station, which includes an area defining an aperature, a Faraday cup and current integrator and a subsystem that loads, holds, and positions the target.
The ion source starts with an approriate molecular species and converts it into ions. The ions are accelerated and then enter the mass analyzer for ion selection. The exit beam of desired implant ions is chosen based on the charge-to-mass ratio of the ions. The ions are then given a final acceleration after which the ion beam will be slightly electrostatically deflected to separate it from any neutral atoms that may have formed. The beam is then scanned over the wafer surface.
The ion source supplies ions, usually singly positively charged, in enough quantity to provide beam currents of from 10 .mu.A to 100 mA, depending on the rating of the implanter. Further, the source must be constructed so that the ions produced can be extracted and formed into a collimated beam. The species to be ionized, which may already be gaseous or be vaporized in or near the source, is confined in a chamber and ionized in a gaseous plasma by impact from electrons. In most cases, the electrons come from a hot filament, and electrons are generated by secondary emission from positive ion bombardment. Also a magnetic field is usually provided to cause spiraling of the electrons, thus increasing the length of the path traveled before the electrons reach the anode and thereby improving ionization efficiency.
Basically, the ion source includes a vaporizer, an arc chamber and a magnet element. Referring to FIG. 1, a conventional arc chamber includes a reaction chamber 10, a filament element 12 used to generate electrons, a first power supplier 14 for providing power to the filament element 12, a second power supplier 16 to create a potential between the filament element 12 and the reaction chamber 10 to increase the ionization efficiency and guide the ions to the extractor, a plurality of gas injection openings 18 to inject suitable gas into the reaction chamber 10 and be ionized in a gaseous plasma by impact from electrons, and filament insulators 20 used for isolation between the filament element 12 and the reaction chamber 10.
As shown in FIG. 2, the filament element 12 and the filament insulators 20 are shown. The filament element 12 is typically surrounded by a first filament insulator 20a and a second filament insulator 20b for isolation. In addition, a set of retainers 20c which are formed of graphite are set between the reaction chamber 10 and the filament insulators 20. FIG. 3a is a cross section view of the first filament insulator 20a. The first filament insulator 20a is an cylinder having both an internal spiral and an external spiral. Further, the first filament insulator 20a has a ring portion 22a at one end. The internal spiral is used to threadably mate with the filament element 12, and the external spiral is used to threadably mate with retainer 20c. The external and internal diameters b1, a1 of the first filament insulator 20a are respectively 0.3, 0.16 and 0.44 inch in length d1. The length c1 of the ring portion 22a is 0.12 inch and 0.5 inch in diameter e1. The first filament insulator 20a has 18 external spiral threads per inch and 32 internal spiral threads per inch. FIG. 3b is a cross section view of the second filament insulator 20b. The second filament insulator 20b is also a cylinder having both an internal spiral and an external spiral. Moreover, the second filament insulator 20b has a first ring portion 22c and a second ring portion 22b at one end. The internal spiral is used to threadably mate with the filament element 12, and the external spiral is used to threadably mate with retainer 20c. The external and internal diameters b2, a2 of the second filament insulator 20b are respectively 0.3, 0.16 and 0.44 inch in length d2. The length c2 of the first ring portion 22c is 0.12 inch. The length e2 of the second ring portion 22b is 0.12 inch and 0.3 inch in diameter. In addition, the second filament insulator 20b has 18 external spiral threads per inch and 32 spiral threads per inch for internal spiral threading.
Typically, a procedure called PM is needed after the arc chamber is used for a period of time. That is, the filament element 12 has to be dismantled from the arc chamber for removing reaction residues coated on the filament element 12. Unfortunately, the first filament insulator 20a and the second filament insulator 20b cannot always be screwed out from the filament element 12 due to the large contact surface with each other. In order to dismantle the filament element 12, the only way is to break down the first filament insulator 20a, and the second filament insulator 20b. In general, the first and second filament insulators 20a and 20b cost U.S. $23.10, and $27.50, respectively. Further, the conventional arc chamber needs a set of graphite retainers for isolation. On the average, the first and second filament insulators 20a and 20b can only be used about 1.5 times. Therefore, this not only raises the cost but also complicates PM.